Method for the connection of two wafers, and a wafer arrangement

ABSTRACT

A method for the connection of two wafers in which a contact area is formed between the two wafers by placing the two wafers one on top of the other. The contact area is heated locally and for a limited time. A wafer arrangement comprises two wafers which have been placed one on top of the other and between whose opposite surfaces a contact area is located. The wafers are connected to one another at selected areas of the contact area.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/068,291 which was filed with the U.S. Patent and Trademark Office onFeb. 28, 2005 now U.S. Pat. No. 7,872,210 which claims priority fromGerman Applications Nos. 10 2004 009 625.2 filed Feb. 27, 2004 and 102004 012 013.7 filed Mar. 11, 2004. The entire disclosures of each ofthese applications are expressly incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a method for the connection of two wafers. Theinvention also relates to a wafer arrangement.

BACKGROUND OF THE INVENTION

A method for the soldering of an electronic component to a dielectricsubstrate is known from the document U.S. Pat. No. 6,284,998 B1. Forthis purpose, at least two connecting points composed of metal areprovided on a surface of the substrate and are each covered with solderpaste. The connections of the electronic component are then brought intocontact with the connecting points on the substrate. The beam of a diodelaser is then directed through that side of the substrate that isopposite the connecting points onto in each case one of the metallicconnecting points until the solder paste on that connecting point melts.The wavelength of the laser beam is in this case selected such that thelaser energy is mainly absorbed by the connecting point and not by thedielectric substrate. Once the connecting point has cooled down, thereis a soldered joint between the connecting point and the connection ofthe electronic component.

Document DE 103 03 978 A1 describes a method for the production of asemiconductor component, in which a thin film semiconductor body issoldered on a mount. For this purpose, solder metals are in each caseapplied to the thin film semiconductor body and to the mount. The thinfilm semiconductor body and the mount are then joined to one another atan increased pressure and at a temperature which is above the meltingpoint of the solder metals involved.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a method forfacilitating the connection of two wafers.

Another object of the invention is to provide a wafer arrangement.

This and other objects are attained in accordance with one aspect of thepresent invention directed to a method for the connection of two wafers.A contact area is formed between the two wafers by placing the twowafers one on top of the other. The connection between the wafers isproduced by heating the contact area on the two wafers locally and for alimited time.

In this context, locally limited heating means that significant heatingof the wafers remains limited in a direction parallel to or at rightangles to the contact area, preferably in both directions. After coolingdown, the two wafers are then mechanically joined to one another intheir contact area, at the location of the local heating.

In one embodiment of the method, two wafers are provided. A material isapplied to at least one of the wafer surfaces. In this case, thematerial can be applied distributed over the entire wafer surface, orthe material can be applied placewise to selected areas of the wafersurface, or the material can be applied distributed over the entirewafer surface and can then be removed from selected areas, for exampleby being etched away. A contact area is then produced between the wafersby placing the wafers one on top of the other such that the appliedmaterial is located between the wafers. A mechanical joint is thenproduced between the wafers by locally, in particular locally limited,heating the material in the contact area for a limited time, said jointbeing effected or mediated by means of the material between the wafers.

In this case, the material may, for example, be selected in a suitableform such that the material is first of all melted by the local heatingand then solidifies during the cooling down process, thereby forming aeutectic with the wafer material.

In a further embodiment of the method, materials are applied to thesurfaces of both wafers. In this case, the material which is applied tothe first wafer is different from the material which is applied to thesecond wafer.

The materials need not necessarily in this case be applied to the entirewafer surface, but can also be applied placewise, to selected areas ofthe wafer surface.

The two wafers are then placed one on top of the other in such a waythat the materials are located between the wafers. The materials arethen heated locally, in particular locally limited, and for a limitedtime in the contact area between the wafers produced in this way, sothat the two different materials are joined in the heated area. This maybe done, for example, by the two materials being melted and by thematerials mixing in the melt. Increased mobility of the particles as aresult of the heating is also feasible, so that a mixture of thematerials occurs on account of particle diffusion.

In any case, after the contact area has been heated locally and for alimited time, a mechanical joint is produced between the wafers, saidjoint being mediated or effected by means of the materials between thewafers.

In one embodiment of the invention, the materials which are applied tothe wafer surfaces in the contact area are solders.

In another embodiment, the solders are solder metals. In this case, thefollowing solder metals are preferably used in the method: Au, AuSn, Pd,In, Pt.

During the local heating of the contact area, these solder metals meltand mix. Once the solder layer has cooled down and solidified, there isthen a mechanical joint between the wafers.

In one embodiment of the method for the connection of two wafers, thecontact area between the wafers is locally heated by means of at leastone laser beam. In this case, it is irrelevant to the principle of themethod whether a single laser beam is used in a time sequence, point bypoint, or, for example, a large number of laser beams are used at thesame time at different points of the contact area.

The wavelength of the laser and at least one of the wafers are in thiscase matched to one another such that at least one of the wafers is atleast partially transparent with respect to the laser beam. This meansthat a small amount of absorption of the energy of the laser beam willtake place, at most, in the wafer.

The laser beam is then focused on the contact area between the wafers,through at least one of the wafers.

In one embodiment, the majority of the laser beam is absorbed by thematerial or the materials at the contact area between the wafers. Thismay be done, for example, by the materials at the contact areapredominantly not being transparent for the laser beam, and absorbingthe energy of the laser beam. This ensures that the contact area isheated, to be precise in a locally limited form around the area at whichthe focus of the laser beam is directed. The power of the laser can inthis case be selected to be sufficiently high that a mechanical joint isproduced between the two wafers at the contact area after cooling down.

In one embodiment of the method, the laser may in this case be operatedin the continuous mode (e.g. in the continuous wave or cw mode).

For one embodiment of the method, a laser in pulsed mode operation isexpedient. In this case, the dissipation of the heat which is producedat the contact area can be optimally set by appropriate selection of thepulse duration and pulse separation. The local limiting of the heatingcan thus be achieved in a particularly simple manner by means of a laserin the pulsed mode. The desired time limiting of the heating is alsoprovided by the limited pulse duration in the case of a laser operatedin the pulsed mode. In this case, an Nd:YAG laser is used for productionof the laser beams in one possible embodiment of the method.

In a further embodiment of the described method, the laser beam ispassed continuously over the entire contact area between the wafers. Thelaser beam may in particular be continuously guided or wielded over theentire contact area between the wafers. In this way, all areas of thecontact area are locally heated in a limited manner, thus resulting inan areal mechanical joint between the two wafers over the entire contactarea of the two wafers. The local heating of the contact area can inthis case be produced successively by means of a single laser beam, orcan be produced at the same time at two or more local areas on thecontact area when using a large number of laser beams.

In another embodiment of the method, the laser is passed over selectedareas of the contact area so that a joint between the two wafers isproduced only in these selected areas. In particular, the laser beam maybe guided or wielded over selected areas of the contact area. In thiscase, a joint is thus produced between the two wafers at selected pointsin the contact area, while other areas of the contact area between thewafers have no joint produced by direct heating. In this case, theshape, arrangement, number and size of the connection areas and of theareas where there is no connection can be configured depending on theproduct requirements.

This means that the shape, arrangement, number and size of theconnecting areas and areas without any connection may, for example, bematched to the required temperature resistance, the preferred mechanicalrobustness, the method of operation of the component, or else to thedesired costs of the product.

In this embodiment of the method as well, it is, of course, possible touse a single laser beam or a large number of laser beams.

In a further embodiment of the method, the joint between the two wafersis produced pointwise at the contact area. For this purpose, the laserbeam is focused only on individual, predetermined points at the contactarea. In this case, a joint is formed between the two wafers at thesepoints. In this case, the individual connection points can be arrangedaccording to a desired pattern. The connection points then form a dotpattern. The number of connection points and the configuration of thenetwork may in this case be matched to the product requirements.

In this embodiment as well, it is possible to use one laser beamsuccessively, or to use a large number of lasers at the same time. Inparticular, it is possible in this case for the number of laser beams tocorrespond to the number of desired connection points.

In one embodiment of the method, a material is applied only to thoseareas of the contact area which are afterwards irradiated by the laserbeam. By way of example, when it is intended to produce a pointwiseconnection between the two wafers, material is applied in advance onlyat these points.

In one embodiment of the described method, at least one of the waferscontains a semiconductor material.

In a further embodiment of the method for the connection of two wafers,at least one of the wafers contains one of the following semiconductormaterials: silicon, germanium, gallium arsenide, InP, GaP.

In one embodiment of the method, at least one of the wafers contains atleast one of the following metals: Mo, Cu, CuW. Furthermore, in afurther embodiment of the method, at least one of the wafers may containceramic materials.

In one embodiment of the method, at least one of the wafers comprisestwo or more individual layers, with at least one of the individuallayers being an epitaxially applied layer.

In a further embodiment of the method, the epitaxially applied layerpreferably contains one of the following semiconductor materials: GaInN,AlGaAs, AlGaInP, GaP, InP, InGaAs, InGaAsP, GaN, AlGaInN.

In this case, in one embodiment of the method, at least one individuallayer of the wafer forms an electronic or microelectronic component.

In this case, in accordance with embodiments of the invention theelectronic component forms an optoelectronic component, for example alight-emitting diode, a semiconductor laser or a detector (for example aphotodiode).

Another aspect of the invention is directed to a wafer arrangement inwhich two wafers which have been placed one on top of the other areconnected to one another at selected areas of their contact area.

This connection may, for example, be effected or mediated by means of amaterial, in particular a single material, or by means two differentmaterials between the wafers.

The wafer arrangement is in this case based on the idea that theconnection between two wafers does not extend areally over the entirecontact area between two wafers, but the two wafers are connected to oneanother only at selected points of their contact area. In this case, thematerials by means of which the connection between the wafers isproduced can either be applied in the entire contact area or only atthose points in the contact area at which a connection between thewafers is located.

In one embodiment of the wafer arrangement, the two wafers are connectedto one another pointwise at connection points at their contact area. Inthis case, the sum of the surface areas of the contact area in which thetwo wafers are connected to one another is small in comparison to thesum of the surface areas of the contact area in which there is noconnection between the two wafers.

In one embodiment of the wafer arrangement, the connection points are inthis case arranged according to a desired pattern.

In one embodiment of the described wafer arrangement, at least one ofthe wafers contains a particularly temperature-sensitive layer. Thismeans that the maximum temperature to which this layer can be heatedwithout damage is lower, for example, than the temperature at which thematerials are joined to one another at the contact area. In this case,heating of the entire wafer arrangement to the temperature at which thematerials are joined would damage the temperature-sensitive layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an outline sketch of the method described here for theconnection of two wafers on the basis of the production of an AlGaInPthin-film light-emitting diode.

FIG. 2 shows a schematic plan view of the contact area of the waferarrangement described here, in which the two wafers are soldered atselected areas in the contact area.

FIG. 3 shows a plan view of the contact area of the wafer arrangementdescribed here, with the two wafers being soldered pointwise.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 explains a method for the production of an AlGaInP thin-filmlight-emitting diode 10. By way of example, a GaAs mount wafer 11 and anepitaxial wafer 12 with an AlGaInP light-emitting diode layer, which isapplied epitaxially to a GaAs substrate, are provided for this purpose.

In one embodiment, two wafers are provided and material is applied tosurfaces of both wafers. By way of example, an AuSn solder layer 13 isapplied to the upper face of the mount wafer 11. By way of example, anAu solder layer 14 is applied to the lower face of the epitaxial wafer12. The solder layers 13, 14 may in this case completely cover thesurfaces of the respective wafers 11, 12, or may be applied only tospecific areas of the wafer surfaces. In this case, it is, of course,irrelevant to the described method which of the solder metals is appliedto which of the two wafers.

In one embodiment, at least one of the wafers 11, 12 may comprise two ormore individual layers. For example, wafer 12 may comprise threeindividual layers 121, 122, and 123, as shown in FIG. 1.

The laser beam 16, for example the beam of an Nd:YAG laser, is thenradiated through the mount wafer 11 at a wavelength at which the mountwafer 11 is transparent with respect to the laser beam 16. In this case,the laser beam 16 is focused on the contact area 15 between the twowafers. Furthermore, however, it is also possible for the laser beam 16to be focused through the epitaxial wafer 12 on the contact layer 15.

The power of the laser is in this case chosen such that the two solderlayers 13, 14 melt locally around the focus of the laser, so that thetwo solders are joined to one another and a local soldered joint isproduced between the two wafers 11 and 12 once the locally heated area17 has cooled down and solidified. The laser may in this case beoperated both in the continuous mode or else in the pulsed mode.

If, during this process, the laser beam is passed continuously over theentire contact area 15, then this results in an areal solder-jointbetween the two wafers 11, 12.

It is self-evident that other mount wafers may also be used in additionto the GaAs wafer that has been mentioned. For example, it is alsopossible to use wafers which include germanium or silicon. If necessary,the wavelength of the laser beam must then be adapted so that the mountwafer is at least partially transparent for the laser beam.

In another embodiment, two wafers are provided but material is appliedto only one of the wafers. In such a case, the material and the otherwafer (i.e., to which the material is not applied) form an eutectic whenthe material is heated in the contact area. One possible combination ofmaterial and wafer is gold and germanium, respectively. This combinationresults in a GeAu eutectic.

There are also numerous options for the choice of the epitaxial wafer.For example, the epitaxial wafer may include a laser diode layer or adetector layer. In particular, the specified method is suitable for thesoldering of temperature-sensitive components, since only a locallylimited area 17 is heated rather than the entire wafer arrangement.

Furthermore, there are no restrictions to the choice of the soldermetals for the described method. Since the thermal load in the describedmethod is locally restricted to the contact area between the wafers,combinations of solder metals, in particular, are feasible which do notproduce a joint until much higher temperatures than the Au and AuSnsolders that have been mentioned.

In case at least one of the wafers comprises a semiconductor materialthe wavelength of the laser is selected according to the band-gap of thesemiconductor material. As is well known for a person of ordinary skillthe art, for wavelengths corresponding to energy smaller than theband-gap the wafer appears transparent. The beam focus is directed tothe contact area and can, therefore, be easily adjusted, for example tothe actual wafer thickness and the distance of laser source and contactarea. Pulse duration and pulse separation depend on the used materials.Adjustments of these parameters is well known to a person with ordinaryskill in the art.

Once the two wafers 11, 12 have been joined, the wafer arrangement canbe separated into individual components—for example individuallight-emitting diode chips. This may be done, for example, by sawing orbreaking of the arrangement. Electrical contact can be made with thefinished component from the side of the mount wafer 11. The solders 13,14 are then preferably electrically conductive.

FIG. 2 shows a schematic plan view of the contact area 15 between twowafers. The two wafers are soldered to one another only at the selectedconnection areas 21 of the contact area 15. In addition to theconnection areas 21, there are also connection-free areas 22 of thecontact area 15, where there is no joint between the two wafers.

In this case, the solder metals may either be applied to the wafer onlyin the connection areas 21, or the solder metals may be applied to theentire contact area 15, distributed over the wafer surfaces.

The shape, size, number and arrangement of the selected connection areas21 and of the connection-free areas 22 may in this case be adapted,depending on the requirements of the wafer arrangement.

FIG. 3 shows a plan view of the contact area 15 on the wafer arrangementdescribed here. In this case, the two wafers are joined to one anotherat junction points 31 of the contact area 15. The junction points 31 arein this case arranged at the nodes of a regular network. Theconnection-free area 32 in this case occupies a far larger surface areaof the contact area 15 than the total surface area of the junctionpoints 31.

In this case, it is either possible to apply the solder metals just tothe junction points 31 on the wafers, or to apply the solder metals tothe wafer surfaces over the entire contact area 15.

The number and arrangement of the junction points 31 are in this casematched to the requirements of the wafer arrangement and to therequirements for the component to be produced. Once the waferarrangement has been separated, components may result in this exemplaryembodiment with which electrical contact is made only at points from theside of the mount wafer 11.

Since the temperature load is particularly low when the two wafers arejoined only at points, this wafer arrangement is particularly suitable,for example, when at least one of the wafers contains atemperature-sensitive component. This is because, when the two wafersare joined at points, the temperature load of the wafers is raisedslightly only at a few points on the wafer arrangement, and the overallrise in temperature load is in this case very low.

In the two last-mentioned exemplary embodiments, it is also possible forone, and only one, component to result per junction point 31 or perconnection area 21. This means that, for example, it is possible for thewafers 11, 12 not to be connected to one another at points at which thewafer arrangement is intended to be separated (for example at weakpoints or on sawing channels). The two wafers 11, 12 are then joinedonly where the components are intended to be produced.

Although heating of the material has been described as being done by alaser beam, it should be understood that other heat sources can be used,such as directing an acoustic ultrasonic wave to the contact area.

The scope of protection of the invention is not limited to the examplesgiven herein above. The invention is embodied in each novelcharacteristic and each combination of characteristics, whichparticularly includes every combination of any features which are statedin the claims, even if this feature or this combination of features isnot explicitly stated in the claims or in the examples.

I claim:
 1. A wafer arrangement, comprising: two wafers placed one ontop of the other; and a contact area between planar surfaces of the twowafers; wherein: the two wafers are connected to one another at selectedconnection areas of the contact area, at least one of the two wafers isa silicon wafer, a germanium wafer, a gallium arsenide wafer, an InPwafer, a ceramic wafer or a GaP wafer, a mechanical joint between thetwo wafers is mediated or effected by a material between the planarsurfaces of the two wafers, wherein said material is applied as acontinuous layer entirely covering the planar surfaces of the two wafersin the contact area, wherein the contact area comprises connection-freeareas arranged therein where there is no mechanical joint between thetwo wafers, at least one of the two wafers comprises two or moreindividual layers, and at least one of the two or more individual layersis an epitaxially applied layer comprising an active layer configured toproduce or to detect radiation, and wherein at least one of the two ormore individual layers is part of an optoelectronic component.
 2. Thewafer arrangement as claimed in claim 1, wherein the mechanical jointbetween the two wafers comprises junction points in the contact area. 3.The wafer arrangement as claimed in claim 2, wherein the junction pointsare arranged at nodes of a regular network.
 4. The wafer arrangement asclaimed in claim 1, wherein one of the wafers is a growth substrate forthe epitaxially applied layer.
 5. The wafer arrangement as claimed inclaim 1, wherein each of the two wafers has the same length in a lateraldirection.
 6. The wafer arrangement as claimed in claim 1, wherein atleast one additional material is applied to one of the wafers, theadditional material being different from the material.
 7. The waferarrangement as claimed in claim 6, wherein the at least one additionalmaterial is solder.
 8. The method as claimed in claim 7, wherein thesolder is selected from the following materials: Au, AuSn, Pd, In andPt.
 9. The wafer arrangement as claimed in claim 6, wherein the at leastone additional material is mixed in a melt.
 10. The wafer arrangement asclaimed in claim 1, wherein at least one of the two wafers istransparent with respect to a laser beam.
 11. The wafer arrangement asclaimed in claim 10, wherein the material is not transparent to thelaser beam.
 12. The wafer arrangement as claimed in claim 1, wherein theepitaxially applied layer comprises at least one of the followingsemiconductor materials: GaInN, AlGaAs, AlGaInP, GaP, InP, InGaAs,InGaAsP, GaN and AlGaInN.
 13. The wafer arrangement as claimed in claim1, wherein at least one of the two wafers comprises at least one of thefollowing materials: Mo, Cu and CuW.
 14. The wafer arrangement asclaimed in claim 1, wherein a sum of the surface areas of the contactarea in which the two wafers are connected to one another is less than asum of the surface areas of the contact area in which there is noconnection between the two wafers.
 15. The wafer arrangement as claimedin claim 1, wherein the two wafers are not connected to one another atseparation points of the wafer arrangement.
 16. A wafer arrangement,comprising: two wafers placed one on top of the other; and a contactarea between planar surfaces of the two wafers, wherein: the two wafersare connected to one another at selected connection areas of the contactarea, and a mechanical joint between the two wafers is mediated oreffected by a material between the planar surfaces of the two wafers,wherein said material is applied as a continuous layer entirely coveringthe planar surfaces of the two wafers in the contact area, wherein thecontact area comprises connection-free areas arranged therein wherethere is no mechanical joint between the two wafers.
 17. The waferarrangement as claimed in claim 16, wherein the mechanical joint betweenthe two wafers comprises junction points in the contact area.
 18. Thewafer arrangement as claimed in claim 17, wherein the junction pointsare arranged at nodes of a regular network.
 19. The wafer arrangement asclaimed in claim 16, wherein at least one of the wafers includes atleast one temperature-sensitive layer, such that a maximum temperatureto which the temperature-sensitive layer can be heated without damage islower than a temperature at which the two wafers are joined together.20. The wafer arrangement as claimed in claim 16, wherein a sum of thesurface areas of the contact area in which the two wafers are connectedto one another is less than a sum of the surface areas of the contactarea in which there is no connection between the two wafers.
 21. Thewafer arrangement as claimed in claim 16, wherein the two wafers are notconnected to one another at separation points of the wafer arrangement.